The invention relates to a method for constructing a superconducting multiphase cable comprising N phases, wherein each phase in the cable is divided into a number of conductors and wherein insulation means are arranged in the cable, the phases being divided into n groups, each group having N different phases.
Superconductor cables utilize the lower resistance in superconductor materials achieved when the superconductor material is exposed to a temperature that is lower than its so-called critical temperature. This temperature may be e.g. 4-7 K (low temperature superconductor, LTS) or 30-10 K high temperature superconductor, HTS). For use at ambient temperature, an artificial refrigerant and a thermal insulation are usually required in order to separate the cable conductor thermally from its surroundings.
Superconductor cables may be produced from a superconducting band wound around a central refrigerating channel. A layer of electrically conducting material is then coaxially applied. A coaxial screen which is either superconducting or normally conducting may then be applied. A thermally insulating layer may be applied either between the inner superconducing layer and the electrical insulation or on the outside of the electrical insulation. Thus, the electrical insulation is either exposed to a high temperature (ambient temperature) or a low temperature (cryogenic temperature). Further, such a cable may have an outer diameter typically in the range from 8 to 15 cm.
The electrical AC loss occurring in the superconductor may be reduced by winding a superconducting band or wire around a refrigerating channel, said winding being made with climbing angles in such a way that an even current distribution among the individual bands/wires is obtained. They may also be wound around several refrigerating channels, e.g. as disclosed in U.S. Pat. No. 4,327,244.
If the current-carrying capacity of cables of this type is to be increased, this may be obtained by increasing the quantity of superconducting material. However, this leads to an increased generation of reactive power, since the reactance/inductance of the cable becomes relatively high, which may cause undesired phase shifts in the conveyed electrical voltage and current, especially in long cable sections but also in short cable sections to which a low voltage is applied and a high current is conducted. Normally, this reactance/inductance may be reduced by increasing the diameter of the inner semiconductor by e.g. 30-50 cm. Even though the reactance/inductance is decreased thereby, this reduction also has drawbacks such as e.g. larger dimensions of the cables, increased consumption of materials, and finally an increased heat transfer because of the increased area of the thermal insulation.
Other methods are disclosed in the literature, which allow reduction of the reactance/inductance of a superconducting cable system. Normally, electrical insulation and electrical screening are included in known cable systems for AC single-phase cables, wherein 3 cables with one phase conductor each are used for providing 3 phases.
From EP 0 786 783 A1 a multiphase superconducting cable is known, in which the individual phases are divided into a number of individual conductors. Each of the individual conductors is isolated from each other and equipped with a superconducting screen. Evidently, this results in a rather expensive and bulky cable, since each individual conductor constitutes a xe2x80x9ccablexe2x80x9d with a conductor, a screen and two layers of insulation.
DE 4340046 A1 discloses a superconducting cable, in which 3 phases are located coaxially in relation to each other and are surrounded by a common return circuit. Reduced consumption of material is thus rendered possible, since 3 phase conductors have a common screen. The diameter of the individual phase conductors may be increased with the purpose of reducing the inductance without an increase of the volume being required which would be the case if 3 individual cables were used. However, the drawback is that a sufficiently reduced inductance will not be obtained, since the relationship between the reactance and the diameter is logarithmic.
Similarly, from JP 123127 a multiphase superconducting cable is known, in which the individual phase conductors consist of a refrigerating channel, a superconducting phase conductor, an electrical insulation, in which each individual phase conductor is surrounded by a normally conducting screen. This reduces the consumption of superconducting material. Since the normally conducting screen is resistive, according to this technique it is necessary to minimize the current induced in these resistive screens by arranging the individual phase conductors in a triangular pattern, in which each individual phase conductor has a different phase as a neighbor.
Both phase divisioning techniques discussed above have the advantage that the reactive power production is reduced, cf. the law of parallel coupling of reactances/inductances. Reducing the electric current in each individual phase conductor also reduces the magnetic field at the surface of the phase conductor and the electrical AC loss in the superconducting material.
Thus, the drawback of these known techniques is that the individual phase conductors consist of complete, independent cables with refrigerating channel, cable conductors, electrical insulation and electrical screen. In practice, it is impossible to produce a compact and inexpensive cable if a high number of groups with a number of phases in each group are desired.
It is an object of the invention to facilitate the manufacturing of a superconducting power cable, preferably for use at 1 kV-132 kV, and superconducting power cable being less bulky, having a higher efficiency and lower manufacturing costs as compared to the output, also for large number of groups. It is a further purpose to reduce the reactance/inductance in a superconducting cable system, without the cable system being bulkier or more expensive than known hitherto.
The object of the invention is fulfilled by a number of N groups of phase conductors being assembled in groups and by one or more of the groups being equipped with a common electrical screen.
In this way, a cable may be manufactured in such a way that the electrical insulation may be produced in essentially one working step which is performed before the various phase conductors are assembled into a cable either by application on the phase conductors of the individual superconductors and/or by production of an electrically insulating foil. Hence, a production is obtained, which is more compact and has lower costs associated thereto, since the number of working operations during production of the cable may be reduced, the individual phases not having individual electrical screens.
For further minimization of the manufacturing costs, it is preferred, as specified in claim 2, that the individual phases only contain superconducting cable wire and an insulation system.
For further simplification of the manufacturing process, the groups may, as specified in claim 3, be arranged in n coaxial groups, either with several different phase conductors in each coaxial layer or with each individual phase conductor in a separate coaxial layer. In this way, a more simple refrigeration system with a limited number of flow paths for refrigerants could be provided.
By arranging the groups in N flat phases as specified in claim 4, the magnetic field generated by current in the phases will be relatively long, so that the magnetic induction in the cable is reduced. In this arrangement, preferably, one or more electrically insulating foil systems may be used as electrical insulation, said foil system(s) consisting of one or more layers of insulating and optionally electrically conducting materials. Use of electrically conducting layers of foil or surface coatings implies that this coating may optionally be removed from selected parts of the foil or it may be selected not to apply this coating on selected parts of the foil.
Further, appropriate embodiments of the invention are specified in the dependent claims.